Engines operating with a variable number of active or deactivated cylinders may be used to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. In some examples, half of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE control system may disable selected cylinders through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves. Variable displacement capabilities can be combined with, for example, variable cam timing (VCT), to further improve fuel economy and emissions performance of the vehicle.
However, a potential issue with variable displacement engines may occur when transitioning operation between the various displacement modes, for example, when transitioning from a non-VDE (or full-cylinder) mode to a VDE (or reduced cylinder) mode, and vice-versa. Specifically, the transitions can significantly affect the manifold pressure, engine airflow, engine power and engine torque output. This can be largely attributed to per-cylinder load changes correlated to the number of activated and deactivated cylinders. Similar engine operating parameters may also be affected by VCT. Thus, for an engine equipped with both VCT and variable displacement capabilities, the systems require coordination during VDE transitions to enable engine torque to meet the driver-demanded torque, while maintaining engine operating conditions within acceptable limits.
One example approach for coordinating VCT and VDE systems is shown by Michelini et al. in U.S. Pat. No. 6,499,449. In this example, VCT, throttle control, and spark retard are used to control manifold air pressure (MAP) during VDE transitions, to thereby maintain a constant driver-demanded torque output. Specifically, during a transition from a reduced cylinder mode to a full cylinder mode, cam timing is retarded to reduce the air charge and MAP provided to the cylinders upon reactivation.
However, the inventors herein have recognized several issues with such an approach, especially during transitions. As one example, the transitions significantly affect the cylinder-specific load of the cylinders by as much as, and possibly more than, 100%, due to per-cylinder load changes correlated to the number of activated and deactivated cylinders. As such, an acceptable cam timing for one mode may lead to excessive residuals in another mode, thus creating a potential for engine misfire during/after a transition. For example, as cylinders are reactivated, the per-cylinder load may decrease to a level where a cam timing acceptable in the partial cylinder mode may be unacceptable in the full cylinder mode, since cylinders can tolerate increased residuals at higher loads. Furthermore, if cam timing is adjusted to affect airflow in order to maintain engine torque, this may exacerbate the situation where residuals are increased even further beyond acceptable levels. For example, if cam timing is further retarded from an already retarded timing when activating cylinders, cylinder misfire may occur immediately after the transition, thus degrading performance. Similar engine operating parameters may also be affected by VCT. Thus, for an engine equipped with both VCT and variable displacement capabilities, the systems require coordination during VDE transitions to enable engine torque to meet the driver-demanded torque, while maintaining engine operating conditions within acceptable limits
Thus, in one example, the above issues may be addressed by a method of operating an internal combustion engine including a variable cam timing (VCT) mechanism in cooperation with a plurality of deactivatable cylinders, each cylinder with a plurality of cylinder valves, the method comprising, operating at a first cylinder valve timing before a transition of reactivating deactivated cylinders, and before the transition, advancing cylinder valve timing from the first valve timing, where after the transition, the cylinder valve timing remains advanced at a second valve timing, the second valve timing advanced relative to the first valve timing.
In one particular example, a variable displacement engine is configured to operate with dual-equal variable cam timing. Herein, prior to cylinder reactivation, that is, during a transition from a reduced cylinder mode to a full cylinder mode, cam timing may be advanced (that is, retarded by a lesser amount) to enable the amount of residuals remaining in the cylinder to be substantially reduced. An acceptable VCT phase angle may be determined based on the engine load, manifold pressure, an estimated barometric pressure, borderline/knock limits, and other engine operating parameters. In this way, the cylinders may be prepared for reactivation so that acceptable levels of residuals are provided during and after the transition, while other parameters compensate for engine torque effects. As such, misfires and partial burns at the time of cylinder reactivation may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.